AMINO-ACID-SEQUENCE OF SPINACH FERREDOXIN-THIOREDOXIN REDUCTASE CATALYTIC SUBUNIT AND IDENTIFICATION OF THIOL-GROUPS CONSTITUTING A REDOX-ACTIVE DISULFIDE AND A [4FE-4S] CLUSTER

Ferredoxin:thioredoxin reductase is a [4Fe-4S] protein involved in the light regulation of carbon metabolism in oxygenic photosynthesis. Thi s enzyme catalyses the reduction of thioredoxins with light generated electrons. Ferredoxin: thioredoxin reductase is composed of two dissim ilar subunits, a catalytic subunit, and a variable subunit. The cataly tic subunit of spinach ferredoxin: thioredoxin reductase, which contai ns the redox-active disulfide bridge, was sequenced by conventional pr otein sequencing techniques and the functional roles of all eight cyst eine residues were examined by chemical modifications. The polypeptide chain with a calculated molecular mass of 12959 Da consists of 113 am ino acids and has a calculated isoelectric point of 5.30. Six of the e ight cysteine residues are clustered as Cys-Pro-Cys and Cys-His-Cys gr oups. Cys19 and Cys27 are free cysteines with no catalytic function, C ys54 and Cys84 constitute the redox-active disulfide bridge of the act ive site, and the remaining four, Cys52, Cys71, Cys73, and Cys82 bind the Fe-S cluster.